A tuna is a saltwater finfish that belongs to the tribeThunnini, a sub-grouping of the mackerel family (Scombridae) – which together with the tunas, also includes the bonitos, mackerels, and Spanish mackerels. Thunnini comprises fifteen species across five genera,[1] the sizes of which vary greatly, ranging from the bullet tuna (max. length: 50 cm (1.6 ft), weight: 1.8 kg (4 lb)) up to the Atlantic bluefin tuna (max. length: 4.6 m (15 ft), weight: 684 kg (1,508 lb)). The bluefin averages 2 m (6.6 ft), and is believed to live for up to 50 years.

Their circulatory and respiratory systems are unique among fish, enabling them to maintain a body temperature higher than that of the surrounding water. An active and agile predator, the tuna has a sleek, streamlined body, and is among the fastest-swimming pelagic fish – the yellowfin tuna, for example, is capable of speeds of up to 75 km/h (47 mph).[2] Found in warm seas, it is extensively fished commercially, and is popular as a game fish. As a result of over-fishing, stocks of some tuna species such as the Southern bluefin tuna have been reduced dangerously close to the point of extinction.[3]

The cladogram is a tool for visualizing and comparing the evolutionary relationships between taxons, and is read left-to-right as if on a timeline. The following cladogram illustrates the relationship between the tunas and other tribes of the family Scombridae. For example, the cladogram illustrates that the skipjack tunas are more closely related to the true tunas than are the slender tunas (the most primitive of the tunas), and that the next nearest relatives of the tunas are the bonitos of the Sardini tribe.[1]

Relative sizes of various tunas, with the Atlantic bluefin tuna (top) at about 8 ft (2.4 m) in this sample

The "true" tunas are those that belong to the genus Thunnus. Until recently, it was thought that there were seven Thunnus species, and that Atlantic bluefin tuna and Pacific bluefin tuna were subspecies of a single species. In 1999, Collette established that based on both molecular and morphological considerations, they are in fact distinct species.[6][7]

The genus Thunnus is further classified into two subgenera: Thunnus (Thunnus) (the bluefin group), and Thunnus (Neothunnus) (the yellowfin group).

The tuna is a sleek and streamlined fish, adapted for speed. It has two closely spaced dorsal fins on its back; The first is "depressible" – it can be laid down, flush, in a groove that runs along its back. Seven to 10 yellow finlets run from the dorsal fins to the tail, which is lunate – curved like a crescent moon – and tapered to pointy tips. The caudal peduncle, to which the tail is attached, is quite thin, with three stabilizing horizontal keels on each side. The tuna's dorsal side is generally a metallic dark blue, while the ventral side, or underside, is silvery or whitish, for camouflage.[37]

Thunnus are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters at latitudes ranging between about 45° north and south of the equator.[38] All tunas are able to maintain the temperature of certain parts of their body above the temperature of ambient seawater. For example, bluefin can maintain a core body temperature of 25–33 °C (77–91 °F), in water as cold as 6 °C (43 °F). However, unlike "typical" endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.[39][40]

Tunas achieve endothermy by conserving the heat generated through normal metabolism. In all tunas, the heart operates at ambient temperature, as it receives cooled blood, and coronary circulation is directly from the gills.[41] The rete mirabile ("wonderful net"), the intertwining of veins and arteries in the body's periphery, allows nearly all of the metabolic heat from venous blood to be "re-claimed" and transferred to the arterial blood via a counter-current exchange system, thus mitigating the effects of surface cooling.[42] This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain,[39][41] which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish.[40]

Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red myotomal muscles derive their color from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish. The oxygen-rich blood further enables energy delivery to their muscles.[39]

For powerful swimming animals like dolphins and tuna, cavitation may be detrimental, because it limits their maximum swimming speed.[43] Even if they have the power to swim faster, dolphins may have to restrict their speed, because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.[43]

Tuna is an important commercial fish. The International Seafood Sustainability Foundation (ISSF) compiled a detailed scientific report on the state of global tuna stocks in 2009, which includes regular updates. According to the ISSF, the most important species for commercial and recreational tuna fisheries are yellowfin (Thunnus albacares), bigeye (T. obesus), bluefin (T. thynnus, T. orientalis, and T. macoyii), albacore (T. alalunga), and skipjack (Katsuwonus pelamis).[38]

The report further states:

Between 1940 and the mid-1960s, the annual world catch of the five principal market species of tunas rose from about 300 thousand tons to about 1 million tons, most of it taken by hook and line. With the development of purse-seine nets, now the predominant gear, catches have risen to more than 4 million tons annually during the last few years. Of these catches, about 68 percent are from the Pacific Ocean, 22 percent from the Indian Ocean, and the remaining 10 percent from the Atlantic Ocean and the Mediterranean Sea. Skipjack makes up about 60 percent of the catch, followed by yellowfin (24 percent), bigeye (10 percent), albacore (5 percent), and bluefin the remainder. Purse-seines take about 62 percent of the world production, longline about 14 percent, pole and line about 11 percent, and a variety of other gears the remainder 3.[38]

The Australian government alleged in 2006 that Japan had illegally overfished southern bluefin by taking 12,000 to 20,000 tonnes per year instead of the their agreed 6,000 tonnes; the value of such overfishing would be as much as US$2 billion.[44] Such overfishing has severely damaged bluefin stocks.[45] According to the WWF, "Japan's huge appetite for tuna will take the most sought-after stocks to the brink of commercial extinction unless fisheries agree on more rigid quotas".[46] Japan's Fisheries Research Agency counters that Australian and New Zealand tuna fishing companies under-report their total catches of southern bluefin tuna and ignore internationally mandated total allowable catch totals.[47]

In recent years, opening day fish auctions at Tokyo's Tsukiji fish market have seen record-setting prices for bluefin tuna, reflecting market demand. In each of 2010, 2011, 2012 and 2013, new record prices have been set for a single fish – the current record is 155.4 million japanese yen (US $1.76 million) for a 221 kg (487 lb) bluefin, or a unit price of JP¥ 703,167/kg (US$ 3,603/lb). The opening auction price for 2014 plummeted to less than 5% of the previous year's price, which had drawn complaints for climbing "way out of line".[48] A summary of record-setting auctions are shown in the following table (highlighted values indicate new world records):

In November 2011, a different record was set when a fisherman in Massachusetts caught an 881-pound tuna. It was captured inadvertently using a dragnet. Due to the laws and restrictions on tuna fishing in the United States, federal authorities impounded the fish because it was not caught with a rod and reel. Because of the tuna's deteriorated condition as a result of the trawl net, the fish sold for just under $5,000.[53]

In 2005 Nauru, defending its vote from Australian criticism at that year's meeting of the International Whaling Commission, argued that some whale species have the potential to devastate Nauru's tuna stocks, and that Nauru's food security and economy relies heavily on fishing.[55] Despite this, Nauru does not permit whaling in its own waters and does not allow other fishing vessels to take or intentionally interact with marine mammals in its Exclusive Economic Zone. In 2010 and 2011 Nauru supported Australian proposals[56] for a western Pacific-wide ban on tuna purse-seining in the vicinity of marine mammals — a measure which was agreed by the Western and Central Pacific Fisheries Commission at its eighth meeting in March 2012.

Dolphins swim beside several tuna species. These include yellowfin tuna in the eastern Pacific Ocean, but not albacore. Tuna schools are believed to associate themselves with dolphins for protection against sharks, which are tuna predators.[57]

Commercial fishing vessels used to exploit this association by searching for dolphin pods. Vessels would encircle the pod with nets to catch the tuna beneath,[58] however the nets were prone to entangling dolphins, injuring or killing them. Public outcry and new government regulations, which are now monitored by NOAA have led to more "dolphin friendly" methods, now generally involving lines rather than nets. However, there are neither universal independent inspection programs nor verification of "dolphin safeness", so these protections are not absolute. According to Consumers Union, the resulting lack of accountability means claims of tuna that is "dolphin safe" should be given little credence.

Fishery practices have changed to be dolphin friendly, which has caused greater bycatch including sharks, turtles and other oceanic fish. Fishermen no longer follow dolphins, but concentrate their fisheries around floating objects such as fish aggregation devices, also known as FADs, which attract large populations of other organisms. Measures taken thus far to satisfy the public demand to protect dolphins can be potentially damaging to other species as well.[59]

Japan is the biggest tuna consuming nation and is also the leader in tuna farming research.[61] Japan first successfully farm-hatched and raised bluefin tuna in 1979. In 2002, it succeeded in completing the reproduction cycle and in 2007, completed a third generation.[62][63][64] The farm breed is known as Kindai tuna. Kindai is the contraction of Kinki University in Japanese (Kinki daigaku).[65] In 2009, Clean Seas, an Australian company which has been receiving assistance from Kinki University[66][67][68] managed to breed Southern Bluefin Tuna in captivity and was awarded the second place in World's Best Invention of 2009 by Time magazine.[69][70]

In the United States, only Albacore can legally be sold in canned form as "white meat tuna";[73] in other countries, yellowfin is also acceptable. While in the early 1980s canned tuna in Australia was most likely Southern bluefin, as of 2003[update] it was usually yellowfin, skipjack, or tongol (labelled "northern bluefin").[71]

As tunas are often caught far from where they are processed, poor interim conservation can lead to spoilage. Tuna is typically gutted by hand, and later pre-cooked for prescribed times of 45 minutes to three hours. The fish are then cleaned and filleted, canned, and sealed, with the dark lateral blood meat often separately canned for pet food. The sealed can is then heated under pressure (called retort cooking) for 2 to 4 hours.[74] This process kills any bacteria, but retains the histamine that can produce rancid flavors. The international standard sets the maximum histamine level at 200 milligrams per kilogram. An Australian study of 53 varieties of unflavored canned tuna found none to exceed the safe histamine level, although some had "off" flavors.[71]

Australian standards once required cans of tuna to contain at least 51% tuna, but these regulations were dropped in 2003.[75][76] The remaining weight is usually oil or water. In the US, the FDA regulates canned tuna (see part c).[77]

Tuna can be a good source of omega-3 fatty acids. It can contain 300 milligrams (0.011 oz) per serving.[78] However, the level of omega-3 oils found in canned tuna is highly variable, since some common manufacturing methods destroy much of the omega-3 oils in the fish.[79] Tuna is also a good source of protein.

Mercury content in tuna can vary widely. For instance, testing by Rutgers University reportedly found that a can of StarKist had 10 times more mercury than another can of similarly identified tuna. This has prompted a Rutgers University scientist whose staff conducted the mercury analysis to say, "That's one of the reasons pregnant women have to be really careful ... If you happen to get a couple or three cans in the high range at a critical period when you are pregnant, it would not be good." Among those calling for improved warnings about mercury in tuna is the American Medical Association, which adopted a policy that physicians should help make their patients more aware of the potential risks.[80]

A study published in 2008 found that mercury distribution in the meat of farmed tuna is inversely related to the lipid content, suggesting that higher lipid concentration within edible tissues of tuna raised in captivity might, other factors remaining equal, have a diluting effect on mercury content.[81] These findings suggest that choosing to consume a type of tuna that has a relatively higher natural fat content might help reduce the amount of mercury intake, compared to consuming tuna with a low fat content.

In 2009 a California appeals court upheld a ruling that canned tuna does not need warning labels as the methylmercury is naturally occurring.[82]

In March 2004, the United StatesFDA issued guidelines recommending that pregnant women, nursing mothers, and children limit their intake of tuna and other predatory fish.[83] The Environmental Protection Agency provides guidelines on how much canned tuna it is safe to eat. Roughly speaking, the guidelines recommend one 6 oz. can of light tuna a week for those weighing less than 110 pounds and two cans a week for those who weigh more.[84]

In 2007 it was reported that some canned light tuna such as yellowfin tuna[85] is significantly higher in mercury than skipjack, and caused Consumers Union and other activist groups to advise pregnant women to refrain from consuming canned tuna.[86]

The Eastern little tuna (Euthynnus affinis) has been available for decades as a low-mercury, less expensive canned tuna. However, of the five major species of canned tuna imported by the United States it is the least commercially attractive, primarily due to its dark color and more pronounced 'fishy' flavor. Its use has traditionally been restricted to institutional (non-retail) commerce.

A January 2008 investigation conducted by the New York Times found potentially dangerous levels of mercury in certain varieties of sushi tuna, reporting levels "so high that the Food and Drug Administration could take legal action to remove the fish from the market."[87]

It is widely accepted that bluefin tuna have been severely overfished, with some stocks at risk of collapse.[96][97] According to the International Seafood Sustainability Foundation (a global, non-profit partnership between the tuna industry, scientists, and the World Wide Fund for Nature), Indian Ocean yellowfin tuna, Pacific Ocean (eastern & western) bigeye tuna, and North Atlantic albacore tuna are all overfished. In April 2009, no stock of skipjack tuna (which makes up roughly 60 percent of all tuna fished worldwide) was considered to be overfished.[98] However, the BBC documentary South Pacific, which first aired in May 2009, stated that, should fishing in the Pacific continue at its current rate, populations of all tuna species could collapse within 5 years.[99] It highlighted huge Japanese and European tuna fishing vessels, sent to the South Pacific international waters after overfishing their own fish stocks to the point of collapse.

A 2010 tuna fishery assessment report, released in January 2012 by the Secretariat of the Pacific Community (SPC), supported this finding, recommending that all tuna fishing should be reduced or limited to current levels and that limits on skipjack fishing be considered.[100]

^ abcCollette B and 8 others (2011). "Thunnus maccoyii". IUCN Red List of Threatened Species. Version 2014.3. International Union for Conservation of Nature. Retrieved 9 January 2015. "This species has been intensively fished since the early 1950s. Its generation length is conservatively estimated to be 12 years. Estimated spawning stock biomass has declined approximately 85% over the past 36 years (1973–2009) and there is no sign that the spawning stock is rebuilding. It is therefore listed as Critically Endangered. Implementation of effective conservation and management measures are urgently needed."

^Cech, J.J.; Laurs, R.M.; Graham, J.B. (1984). "Temperature-induced changes in blood gas equilibria in the albacore, Thunnus alalunga, a warm-bodied tuna" (PDF). Journal of experimental biology109 (1): 21–34. Oxygenated blood that has just reached thermal equilibrium with ambient sea water in the gills enters the rete on the arterial side, while warmed, deoxygenated, and carbon dioxide-laden blood enters on the venous end. In the rete, countercurrent flow and the high surface area contact between the two blood supplies facilitate the transfer of nearly all of the metabolic heat in the venous blood to arterial blood, thus conserving muscle temperature. After exiting the rete, arterial blood continues to the red muscle capillary beds, and cooled venous blood flows to the gills where carbon dioxide is excreted and oxygen is loaded.

^ abIosilevskii, G; Weihs, D (6 March 2008). "Speed limits on swimming of fishes and cetaceans". Journal of The Royal Society Interface5 (20): 329–338. doi:10.1098/rsif.2007.1073. Retrieved 13 July 2014. Lacking pain receptors on their caudal fins, scombrids may temporarily cross the cavitation limit, and cavitation-induced damage has been observed (Kishinouye 1923); on the other hand, delphinids probably cannot cross it without pain (Lang 1966)